Optical information recording and reproducing apparatuses are capable of recording and reproducing such information as music, images, and data on and from an optical disc.
The development of optical discs has recently reached commercialization of Blu-ray Disc (BD) which employs a blue semiconductor laser and a high-NA objective lens. This new technology, however, seems to have reached its limit in increasing the recording capacity through improvement of the resolving power of the optical system. A possible way to go beyond this limit is by multiplication of information recording layers in the optical disc. Unfortunately, the multiplayer optical disc has a problem with interlayer cross talk between layers which adversely affects servo signals. The interlayer cross talk occurs because the photodetector receives reflected stray light from out-of-focus layers not engaged in recording and reproducing.
The optical disc apparatus is designed such that the reflected light from the optical disc is divided and diffracted into multiple light beams by a diffracting optical element, such as HOE (Holographic Optical Element), and the resulting light beams are received by a photodetector and the signals detected in response to the amount of light received generate reproducing RF signal (RFS: Radio Frequency Signal), focus error signal (FES: Focus Error Signal), and tracking error signal (TES: Tracking Error Signal). The detection of TES is accomplished by push-pull method (PP: Push-Pull) for the recoding type disc of BD-R (Blu-ray Disc-Recordable) and BD-RE (REwritable) or by Differential Phase Detection (PDP) method for BD-ROM (Read Only Memory). The signal detection and signal processing are switched according to the type of recording medium in use.
Japanese Patent Application No. 2006-283248, for example, discloses the use of the knife edge method for detection of focus error signals in the case of dual layer optical disc. According to this disclosure, the photodetector is arranged in such a way that reflected stray light from the out-of-focus layer does not illuminate the photodetector to detect servo signals. In order that the reflected stray light from the out-of-focus layer illuminates as little as possible the photodetector for servo signals, the diffracted light of first order or minus first order is detected mainly to generate RFS.
Also, Japanese Patent Application No. 2007-289894, for example, discloses the method for removing the stray light component that has illuminated the photodetector. According to this disclosure, the main photodetector to detect the reflected light from the optical disc, thereby generating RFS, TES, and FES, is juxtaposed with a secondary photodetector which detects only the reflected stray light from the out-of-focus layer. The secondary photodetector calculates stray light signals of the same degree as the reflected stray light received by the main photodetector so as to give a difference from the detected signal. In this way it is possible to detect stable TES free of stray light signal component from the target layer. The method disclosed in Japanese Patent Application No. 2007-289894 employs SSD (Spot Size Detection) for detection of focus error signals. The photodetector for SSD is positioned away from the photodetector for RFS and TES so that it does not receive the reflected stray light from the out-of-focus layer.